Description of the Related Art
In recent years research has been conducted into techniques for forming transistors which employ thin-film semiconductors on a glass or quartz substrate (referred to as thin-film transistors). In particular, techniques employing amorphous silicon as the thin-film semiconductor have been put to practical use, for use in active matrix-type liquid crystal display devices and the like.
However, thin-film transistors which employ amorphous silicon have the problem that their characteristics are poor. For example, if an improvement is required in the display function of an active matrix-type liquid crystal display device, the characteristics of thin-film transistors which employ amorphous silicon are too poor to achieve this.
Further, techniques are known for constructing thin-film transistors using crystalline silicon films in which amorphous silicon films have been crystallized. These techniques involve transforming an amorphous silicon film into a crystalline silicon film by performing heat treatment or irradiation with laser light after formation of the amorphous silicon film. Crystalline silicon films obtained by crystallizing amorphous silicon films generally have a multicrystalline construction or a microcrystalline construction.
By constructing thin-film transistors using crystalline silicon films it is possible to obtain much better characteristics than if amorphous silicon films are used. For example, considering the mobility, which is one index with which to evaluate the characteristics of thin-film transistors, with thin-film transistors employing amorphous silicon films the mobility is 1 cm.sup.2 /Vs or less, but with thin-film transistors employing crystalline silicon films, a value of approximately 100 cm.sup.2 /Vs can be achieved.
However, crystalline silicon films which are obtained by crystallizing amorphous silicon films have a multicrystalline construction, and there are a number of problems which result from crystal grain boundaries. For example, since some carriers migrate via crystal grain boundaries, there is the problem that voltage resistance is greatly limited. There is the further problem that under high speed operation, for example, variation and degradation of the characteristics is liable to occur. There is furthermore the problem that since some carriers migrate via crystal grain boundaries, there is a large leak current when the thin-film transistor is off.
Further, in order to construct active matrix-type liquid crystal display devices in a more integrated fashion it is desirable to form not only the pixel region but also the peripheral circuitry on a single glass substrate. In such cases the thin-film transistors which are arranged in the peripheral circuitry must be able to handle large currents in order to drive the many thousands of pixel transistors which are arranged in matrix form.
In order to obtain thin-film transistors which can handle large currents it is necessary to adopt a construction which has a wide channel. However, with thin-film transistors which employ polycrystalline silicon films or microcrystalline silicon films there is a problem that this cannot be realized even if the channel is widened, due to the problem of voltage resistance. There is the further problem that variations in the threshold voltage and the like are large, and they are therefore not practical.